This application is based on Japanese Patent Applications Nos. 2000-95247 and 2000-368343, filed on Mar. 29, 2000 and Dec. 4, 2000, respectively, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to an optical device, or a taking lens device. More specifically, the present invention relates to a taking lens device that optically takes in an image of a subject through an optical system and that then outputs the image as an electrical signal, for example, a taking lens device that is used as a main component of a digital still camera, a digital video camera, or a camera that is incorporated in or externally fitted to a device such as a digital video unit, a personal computer, a mobile computer, a portable telephone, or a personal digital assistant (PDA). The present invention relates particularly to a taking lens device which is provided with a compact, high-zoom-ratio zoom lens system.
2. Description of Prior Art
In recent years, as personal computers and other data processing devices have become more and more popular, digital still cameras, digital video cameras, and the like (hereinafter collectively referred to as digital cameras) have been coming into increasingly wide use. Personal users are using these digital cameras as handy devices that permit easy acquisition of image data to be fed to digital devices. As image data input devices, digital cameras are expected to continue gaining popularity.
In general, the image quality of a digital camera depends on the number of pixels in the solid-state image sensor, such as a CCD (charge-coupled device), which is incorporated therein. Nowadays, many digital cameras which are designed for general consumers, boast of high resolution of over a million pixels, and are thus approaching silver-halide film cameras in image quality. On the other hand, even in digital cameras designed for general consumers, zoom capability (especially optical zoom capability with minimal image degradation) is desired, and therefore, in recent years, there has been an increasing demand for zoom lenses for digital cameras that offer both a high zoom ratio and high image quality.
However, conventional zoom lenses for digital cameras that offer high image quality of over a million pixels are usually built as relatively large lens systems. One way to avoid this inconvenience is to use, as zoom lenses for digital cameras, zoom lenses which were originally designed for lens-shutter cameras in which remarkable miniaturization and zoom ratio enhancement have been achieved in recent years. However, if a zoom lens designed for a lens-shutter camera is used unchanged in a digital camera, it is not possible to make good use of the light-condensing ability of the microlenses disposed on the front surface of the solid-state image sensor. This causes severe unevenness in brightness between a central portion and a peripheral portion of the captured image. The reason is that in a lens-shutter camera, the exit pupil of the taking lens system is located near the image plane, and therefore off-axial rays exiting from the taking lens system strike the image plane from oblique directions. This can be avoided by locating the exit pupil away from the image plane, but not without making the taking lens system larger.
An object of the present invention is to provide an optical, or a taking lens device, which is provided with a novel zoom lens system that, despite being compact, offers both a high zoom ratio and high image quality.
To achieve this object, according to one aspect of the present invention, an optical, or taking lens device is provided with: a zoom lens system that is comprised of a plurality of lens units which achieves zooming by varying the unit-to-unit distances; and an image sensor that converts an optical image formed by the zoom lens system into an electrical signal. The zoom lens system comprises at least, from the object side thereof to an image side thereof, a first lens unit having a negative optical power, a second lens unit having a negative optical power, a third lens unit having a positive optical power, and a fourth lens unit having a positive optical power. The zoom lens system achieves zooming by varying the distances between the first to fourth lens units.
According to another aspect of the present invention, an optical, or taking lens device is provided with: a zoom lens system that is comprised of a plurality of lens units which achieves zooming by varying the unit-to-unit distances; and an image sensor that converts an optical image formed by the zoom lens system into an electrical signal. The zoom lens system is comprised of, at least from the object side, a first lens unit having a negative optical power, a second lens unit having a negative optical power, and a third lens unit having a positive optical power. The first lens unit comprises a single lens element.